Thanks for the responses. You both bring up some good points. Brian you have identified the heat issue and its effect on shingles and problems with ice damming. Ice damming problems are true for both stress-skin and conventional roofs. One effective technique is to apply a layer of bituminous "paper" such as "Ice and Water Shield" on the lower 3 feet of the roof, instead of building felt. By the way, felt should not be used at all under fiberglas shingles (on panels) because it deforms excessively from the "hot roof" condition.
Rudy points out the problem of the "chimney effect", while this effect is precisely what we are trying to create, conversely it brings with it the hazard of fire promotion. If fire managed to get into an eave, the convection of air and flame through our intentional vents from eave to ridge would cause rapid spread of the fire. This problem is exacerbated in ventilated panels where toxic gasses can be given off, and structural integrity more rapidly lost from the melting foam.
Ventilated panels? I have literature from Branch River printed in 1987 advertizing their Air-Flo Roof Panel which contains air channels under the upper layer of osb. Rudy mentioned Cornell Corp.'s ThermaCal and I expect there are more. Winter Panel recommends a "cold" (ventilated) roof. However some shingle manufacturers have begun including their warranty on panel roofs, such as Elk Corporation.
According to the Journal of Light Construction, Bill Rose of the University of Illinois has run a ten year side-by-side study with no noticeable difference. He has found however that by the time the air reaches the top of the ventilation channel it is so hot that their is little to no cooling effect on the upper shingles.
If a client wants a shingle roof, but are concerned about the life of the shingles, I recommend that they use an "Architectural Grade" shingle, light in color. I suspect that if one invested the money that it would take to build a "second roof", in 20-30 years at even a modest interest rate, there will be enought money to replace the shingles with a superior roof material . . . . whatever that is in 2020 . . . . probably a silicon, self healing, solar collecting shingle, with built in satellite signal receivers, and chameleon pigment displaying your favorite screen saver on the hour!
[This message has been edited by Paul Freeman (edited 03-05-99).]
